This invention relates to a process for preparing polymers of prochiral olefins, especially vinylaromatic monomers. The resulting polymers have a syndiotactic stereostructure. More particularly, the present process allows for the preparation of such polymers in high efficiency and having a desired low molecular weight less than 500,000.
It is previously known in the art to prepare highly syndiotactic vinyl aromatic polymers and other prochiral olefins by the use of titanium complexes containing a single cyclopentadienyl or substituted cyclopentadienyl group and an activating cocatalyst including alkylalumoxanes, inert, noncoordinating ion forming compounds, Lewis acids and mixtures thereof. Disclosures of such prior art processes are found in U.S. Pat. Nos. 5,045,517, 5,066,741, 5,206,197 and WO 92/05823 (equivalent to U.S. Ser. No. 07/740529, filed Aug. 5, 1991. The teachings of all of the foregoing patents and patent applications and publications is hereby incorporated by reference.
Generally, the preparation of lower molecular weight polymers by means of a Ziegler-Natta metallocene catalyst is more difficult than the preparation of higher molecular weight polymers. Previously utilized polymerization processes have controlled the molecular weight of the resulting polymer by the use of a variety of chain transfer agents which interrupt the polymerization process causing the formation of lower molecular weight polymers. In the preparation of syndiotactic polymers of vinylaromatic monomers, chain transfer agents normally used in aliphatic olefin polymerizations have proven ineffective or inconvenient to use in the desired reaction equipment. In the absence of chain transfer agents, syndiotactic olefin polymers of molecular weight greater than 500,000 and often greater than 600,000 are generally prepared. Such polymers are difficult to process efficiently without undesired decomposition due to high temperatures. Consequently, there exists a need in the art for an efficient, highly productive process to prepare syndiotactic olefin polymers, particularly, syndiotactic vinyl aromatic polymers, having reduced molecular weight.